New Delhi, Jan 23 : By 2050, most people on Earth will live downstream of tens of thousands of large dams built in the 20th century, many of them including Indias already operating at or beyond their design life, putting lives and property at risk, a UN University (UNU) analysis revealed. Besides India, the analysis also includes dam decommissioning or ageing case studies from the US, France, Canada, Japan, and Zambia and Zimbabwe. The report, "Ageing water infrastructure: An emerging global risk", by UNU's Canadian-based Institute for Water, Environment and Health, released on Friday said most of the 58,700 large dams worldwide were constructed between 1930 and 1970 with a design life of 50 to 100 years, adding that at 50 years, a large concrete dam "would most probably begin to express signs of ageing". Ageing signs include increasing cases of dam failures, progressively increasing costs of dam repair and maintenance, increasing reservoir sedimentation, and loss of a dam's functionality and effectiveness, "strongly interconnected" manifestations, the paper says. The report says dams that are well designed, constructed and maintained can "easily" reach 100 years of service but predicts an increase in "decommissioning", a phenomenon gaining pace in the US and Europe, as economic and practical limitations prevent ageing dams from being upgraded or if their original use is now obsolete. Worldwide, the huge volume of water stored behind large dams is estimated at 7,000 to 8,300 cubic kilometres, enough to cover about 80 per cent of Canada's landmass under a metre of water. The report provides an overview of dam ageing by world region and primary function -- water supply, irrigation, flood control, hydropower, and recreation. It also details the increasing risk of older dams, the rising maintenance expense, the declining functionality due to sedimentation, the benefits of restoring or redesigning natural environments, and the societal impacts, pros and cons that need to be weighed by policy makers deciding what to do. Notably, "the nature of these impacts varies significantly between low- and high-income countries". It says the climate change will accelerate the dam ageing process. "This report aims to attract global attention to the creeping issue of ageing water storage infrastructure and stimulate international efforts to deal with this emerging, rising water risk," says co-author Vladimir Smakhtin, Director of UNU-INWEH. "Underlined is the fact that the rising frequency and severity of flooding and other extreme environmental events can overwhelm a dam's design limits and accelerate a dam's ageing process. Decisions about decommissioning, therefore, need to be taken in the context of a changing climate." Lead author and UNU-INWEH Senior Researcher Duminda Perera, said: "This problem of ageing large dams today confronts a relatively small number of countries, 93 per cent of the world's large dams are located in just 25 nations. "Large dam construction surged in the mid-20th century and peaked in the 1960s-70s, especially in Asia, Europe and North America, while in Africa the peak occurred in the 1980s. The number of newly-constructed large dams after that continuously and progressively declined." According to the report, the world is unlikely to witness another large dam building revolution as in the mid-20th century, but dams constructed then will inevitably be showing their age. China has 23,841 large dams (40 per cent of the world's total). And 32,716 large dams (55 per cent of the world's total) are found in just four Asian countries: China, India, Japan, and the South Korea, a majority of which will reach the 50-year threshold relatively soon. The same is true of many large dams in Africa, South America and Eastern Europe. The pace of large dam construction has dropped dramatically in the last four decades and continues to decline in part because "the best locations for such dams globally have been progressively diminishing as nearly 50 per cent of global river volume is already fragmented or regulated by dams", the report says. As well, there are strong concerns regarding the environmental and social impacts of dams, and large dams in particular, as well as emerging ideas and practices on the alternative types of water storage, nature-based solutions, and types of energy production beyond hydropower. Public safety, escalating maintenance costs, reservoir sedimentation, and restoration of a natural river ecosystem are among the reasons driving dam decommissioning. Decommissioning will also have various positive and negative economic, social, and ecological impacts to be considered in a local and regional social, economic, and geographic context "critical to protect the broader, sustainable development objectives for a region", the report says. In addition to the three UNU-INWEH experts, the report was co-authored by Spencer Williams, Graduate Institute of International and Development Studies, Geneva, Switzerland, and Taylor North of McMaster University, Hamilton, Canada. /IANS